Several methods are known for measuring the thicknesses of coatings supported by substrates. Some of these methods make use of:                measuring the shape of workpieces, before and after applying the coating to the substrate; or        measuring the magnetic permeability of the substrate by means of a probe placed on the coating and consequently spaced apart from the substrate by a distance corresponding to the thickness of the coating (acting as a magnetic circuit gap and sometimes referred to as “lift-off”); or        using eddy currents generated in the substrate to perform the measurements, providing the coating is non-conductive while the substrate is itself conductive.        
Nevertheless, those methods have drawbacks. The first above-mentioned method cannot measure accurately the thickness of the coating carried by a cylindrical surface, e.g. when the substrate is in the form of a body of revolution such as a tube for airplane landing gear. Measuring the diameter of the coated cylinder gives an idea about the sum of the thicknesses of the coatings on the opposite sides of the cylinder, but it does not make it possible to know the individual values of each of those thicknesses. With such a method, it is not possible to guarantee that the coating formed on the cylindrical substrate does indeed have a minimum thickness as required by standards.
When coating a material that is electrically conductive, such as tungsten carbide (WC—Co—Cr) placed on a substrate that is conductive and non-magnetic, such as titanium, neither of the last two methods gives satisfaction.
The second method cannot be used when the substrate is non-magnetic, since the magnetic field used in measuring magnetic permeability is not disturbed by titanium. It is therefore not possible to detect any variation in the magnetic field as a function of the distance of the probe from the substrate, so is it not possible to deduce therefrom the thickness of the coating layer.
The use of eddy currents for detecting the thickness of an electrically conductive coating carried on a substrate that is also conductive lacks accuracy and it is desirable to improve such accuracy.